The present invention relates, in general, to linepipe and more particularly to low yield ratio, dual phase steel linepipe having a superior strain aging resistance and methods for making the same.
Natural gas is becoming an increasingly important energy source. Often the major natural gas fields in the world are far removed from major markets. As such, pipelines may have to traverse long distances over land or under water, which can cause severe strains on the pipeline. Seismically active regions and arctic regions that are subject to frost-heave and thaw settlement cycles can cause severe strains on a pipeline. Pipelines laid across sea beds also experience severe strains due to displacement or bending caused by water currents.
Accordingly, linepipe used for these environments requires excellent strain capacity, such as excellent uniform elongation and a low yield to tensile strength ratio or yield ratio (YR) in the longitudinal direction of pipe to ensure mechanical integrity. Dual phase (DP) steel has a relatively soft phase, such as a ferrite phase and a relatively hard phase. The harder phase usually has more than one constituent. Dual phase steels (i.e. steel having a dual phase (DP) microstructure) offer high uniform elongation and low yield ratios and thus, provide superior strain capacity. For these reasons, DP steel linepipe is attractive for installation in seismically active areas or in arctic regions subject to semi-perma frost conditions or in other situations which demand high strain capacity.
DP steel is typically processed according to a series of steps. For example, a steel slab is typically re-heated to an austenite temperature range of about 1,000° C. to 1250° C., and rough rolled within a recrystallization temperature range to refine the grain size. The rough-rolled steel is then finish rolled within a non-recrystallization temperature range, and cooled to a temperature below Ar3 to form ferrite followed by accelerated cooling to a temperature of 400° C. or less. The plate is then typically worked into a U shape, then an O shape, seam welded, and expanded (known as UOE pipe making process) to the desired outer diameter. Arc welding, resistance welding or laser welding or the like can be used for the seam welding step in the UOE process.
Afterwards, the outer diameter of the pipe is typically coated to provide protection against corrosion. Fusion bonded epoxy (FBE) coating is typically used for this purpose. During the FBE coating process, the pipe is heated to an elevated temperature and coated with a polymer.
Due to the linepipe fabrication and coating processes, most linepipe steels including DP steels are susceptible to strain aging. Strain aging leads to a degradation of strain capacity, and is a type of behavior, usually associated with yield point phenomena, in which the flow or yield strength of a metal is increased and the ductility is decreased upon heating after cold working, such as during the FBE coating process. In other words, strain aging refers to the hardening of metals with a corresponding decrease in ductility.
Strain aging can be caused by the interaction between the stress field of dislocations and the strain field of solute atoms in the steel. The formation of solute atmospheres (“Cottrell atmospheres”) around dislocations increases the resistance to dislocation movement on subsequent loading. Ductility in metals is generally proportional to the ease with which dislocations move in that metal. As a result, higher forces or stress is required to tear the dislocations away from the Cottrell atmospheres, leading to an increase of yield strength, loss of ductility and increase in ductile-to-brittle transition temperature. The net result is that strain aging reduces strain capacity. A steel or component fabricated out of that steel with higher resistance to strain aging will therefore substantially retain its strain capacity after aging following cold working.
The aging process is thought to occur in two stages. In the first stage, solute species diffuse to the dislocations to form atmospheres. In the second stage, the solute species form precipitates on the dislocations. Those precipitates contribute to the overall strength increase of the material but lower the elongation to fracture. Often, only the first stage occurs if there is a low concentration of solute species.
The elements typically responsible for relatively low temperature (≦300° C.) strain aging in steel are carbon and nitrogen, which are interstitial solute elements in steel. Those elements have low equilibrium solubility and significantly higher diffusivities compared to that of substitutional solutes within the steel, such as chromium, vanadium, molybdenum, copper, and magnesium, just to name a few. However, carbide and nitride forming substitutional alloying elements such as chromium, vanadium, molybdenum, etc. can have an indirect effect on strain aging susceptibility by increasing the equilibrium solubility of carbon and nitrogen. As such, solute carbon and nitrogen have a tendency to migrate to dislocations in the ferrite phase forming the Cottrell atmospheres. As mentioned above, these Cottrell atmospheres tend to restrict the motion of dislocations and therefore, compromise the strain capacity of the steel.
Similarly, it is believed that the yield strength of dual phase steel linepipe can be increased during post-formation treatments, such as the FBE coating process. As mentioned, a typical FBE coating process requires heat. The thermal exposure required by the FBE coating process provides enough energy for the solid solution carbon and/or nitrogen atoms in the linepipe to migrate to the dislocations in the ferrite phase. That migration compromises the strain capacity of the linepipe for the reasons stated above.
There is a need, therefore, for a dual phase steel and linepipe made therefrom that have a low yield ratio, high uniform elongation and excellent work hardening properties for high strain capacity to ensure mechanical integrity in aggressive environment applications. There is also a need for new steel chemistries that will impart excellent strain aging resistance to the steel and products fabricated therefrom. Further, there is a need for a method to process dual phase steel, which provides superior strain aging resistance attributes in the linepipe and the precursor steel from which it is fabricated for excellent strain capacity, particularly after a thermal treatment process such a FBE coating process.